Rhodium-Catalysed Hydrogenations Using Monodentate Ligands

The use of monodentate phosphorus ligands, such as phosphonites, phosphites and phosphoramidites, in the rhodium-catalysed asymmetric hydrogenation of a range of mostly alkene type substrates was reported for the first time in 2000. Not only are these ligands cheap and easy to prepare in one or two steps, their use has also created new opportunities, such as their robotic parallel synthesis and the use of complexes containing two different monodentate ligands, which tremendously increases the available diversity. This review covers the period between 2006 and 2016. Many new ligands have been made during this time; not only new variants on the three ligand types that were earlier reported, but also monodentate phosphines and secondary phosphine oxides. These were mostly tested on the usual N-acetyl-dehydroamino acids, itaconic esters and enamide type substrates. Other more novel substrates were N-formyl-dehydroamino acids, all the variants of the beta-dehydroamino acid family, enol esters, 2-methylidene-1,2,3,4-tetrahydro-β-carbolines, alkenes containing phosphonate or thioether substituents, several substituted acrylic acids as well as substituted cinnamic acids. The mechanism of the rhodium-catalysed hydrogenation with phosphites, phosphonites, phosphoramidites as well as phosphepines has been reported. A common theme in these mechanisms is the formation of a dimeric bimetallic complex after subjecting the [RhL2(cod)]X or [RhL2(nbd)]X (X = BF4,PF6, SbF6) complexes to hydrogen. Since these hydrogenations are usually carried out in non-polar solvents, the formation of the expected RhL2(Solvent)2 complexes does not occur after the removal of the diene and instead each rhodium atom in these dimeric complexes coordinates not only to two monodentate ligands, but also in η6 fashion to an aromatic ring of one of the ligands that is bound to the other rhodium atom. These complexes can react with the substrate to form the substrate complex that is hydrogenated. Other studies also found that it is possible to form rhodium hydride complexes first, which react with the substrate to form product. There is one well-described industrial application on large scale in which a substituted 2-isopropyl-cinnamic acid is hydrogenated using a rhodium complex with a mixture of 2 eq. of 3,3'-dimethyl-PipPhos and 1 eq. of triphenylphosphine. The addition of the non-chiral triarylphosphine not only accelerated the reaction 50-fold, also the enantioselectivity was much improved. The product was used as a building block for AliskirenTM, a blood-pressure lowering agent.